Silicon carbide epitaxial substrate and method for manufacturing silicon carbide semiconductor device

ABSTRACT

A silicon carbide epitaxial substrate includes a silicon carbide single crystal substrate and a silicon carbide layer. The silicon carbide single crystal substrate has a first main surface. The silicon carbide layer is on the first main surface. The silicon carbide layer includes a second main surface opposite to a surface thereof in contact with the silicon carbide single crystal substrate. The second main surface has a maximum diameter of more than or equal to 100 mm. The second main surface includes an outer peripheral region which is within 3 mm from an outer edge of the second main surface, and a central region surrounded by the outer peripheral region. The central region has a haze of less than or equal to 75 ppm.

TECHNICAL FIELD

The present disclosure relates to a silicon carbide epitaxial substrateand a method for manufacturing a silicon carbide semiconductor device.The present application claims priority to Japanese Patent ApplicationNo. 2015-202012 filed on Oct. 13, 2015, the disclosure of which isincorporated herein by reference in its entirety.

BACKGROUND ART

Japanese Patent Laying-Open No. 2013-34007 (PTD 1) discloses a siliconcarbide epitaxial substrate characterized by having no short stepbunching.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2013-34007

SUMMARY OF INVENTION

A silicon carbide epitaxial substrate in accordance with the presentdisclosure includes a silicon carbide single crystal substrate and asilicon carbide layer. The silicon carbide single crystal substrate hasa first main surface. The silicon carbide layer is on the first mainsurface. The silicon carbide layer includes a second main surfaceopposite to a surface thereof in contact with the silicon carbide singlecrystal substrate. The second main surface has a maximum diameter ofmore than or equal to 100 mm. The second main surface includes an outerperipheral region which is within 3 mm from an outer edge of the secondmain surface, and a central region surrounded by the outer peripheralregion. The central region has a haze of less than or equal to 75 ppm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view showing a configuration of a siliconcarbide epitaxial substrate in accordance with the present embodiment.

FIG. 2 is a schematic cross sectional view showing the configuration ofthe silicon carbide epitaxial substrate in accordance with the presentembodiment.

FIG. 3 is a schematic plan view showing positions for measuring acarrier concentration.

FIG. 4 is a schematic cross sectional view taken along a line IV-IV inFIG. 5 (on the left side) and a schematic cross sectional view takenalong a line IV-IV in FIG. 6 (on the right side).

FIG. 5 is a schematic plan view showing a shallow pit.

FIG. 6 is a schematic plan view showing a deep pit.

FIG. 7 is a schematic cross sectional view showing configurations of theshallow pit (on the left side) and the deep pit (on the right side).

FIG. 8 is a schematic plan view showing a configuration of a firstexample of the deep pit.

FIG. 9 is a schematic plan view showing a configuration of a secondexample of the deep pit.

FIG. 10 is a schematic plan view showing a configuration of a thirdexample of the deep pit.

FIG. 11 is a schematic see-through side view showing a configuration ofa device for manufacturing the silicon carbide epitaxial substrate inaccordance with the present embodiment.

FIG. 12 is a timing chart showing an example of condition control duringepitaxial growth.

FIG. 13 is a timing chart showing details of the condition controlduring the epitaxial growth.

FIG. 14 is a schematic plan view showing a first example of aconfiguration in the vicinity of a susceptor plate.

FIG. 15 is a schematic cross sectional view showing a second example ofthe configuration in the vicinity of the susceptor plate.

FIG. 16 is a schematic plan view showing a configuration of atrapezoidal defect.

FIG. 17 is a schematic cross sectional view taken along a line XVII-XVIIin FIG. 16.

FIG. 18 is a schematic cross sectional view taken along a lineXVIII-XVIII in FIG. 16.

FIG. 19 is an enlarged view of a region XIX in FIG. 16.

FIG. 20 is a schematic plan view showing a configuration of a triangulardefect.

FIG. 21 is a flowchart schematically showing a method for manufacturinga silicon carbide semiconductor device in accordance with the presentembodiment.

FIG. 22 is a schematic cross sectional view showing a first step of themethod for manufacturing the silicon carbide semiconductor device inaccordance with the present embodiment.

FIG. 23 is a schematic cross sectional view showing a second step of themethod for manufacturing the silicon carbide semiconductor device inaccordance with the present embodiment.

FIG. 24 is a schematic cross sectional view showing a third step of themethod for manufacturing the silicon carbide semiconductor device inaccordance with the present embodiment.

FIG. 25 is a Weibull plot showing the relation between acharge-to-breakdown (Q_(BD)) and a cumulative failure rate (F).

FIG. 26 is a view showing the relation between a haze and thecharge-to-breakdown (Q_(BD)).

DESCRIPTION OF EMBODIMENTS Description of Embodiment of PresentDisclosure

In the description below, identical or corresponding elements will bedesignated by the same reference numerals, and the same descriptionthereof will not be repeated. Regarding crystallographic indications inthe present specification, an individual orientation is represented by [], a group orientation is represented by < >, an individual plane isrepresented by ( ), and a group plane is represented by { }. Generally,a negative index is supposed to be crystallographically indicated byputting “-” (bar) above a numeral, but is indicated by putting thenegative sign before the numeral in the present specification. Further,in the description below, regarding crystal planes of silicon carbide(SiC), a (000-1) plane may be referred to as a “C (carbon) plane”, and a(0001) plane may be referred to as a “Si (silicon) plane”.

(1) A silicon carbide epitaxial substrate 100 in accordance with thepresent disclosure includes a silicon carbide single crystal substrate10 and a silicon carbide layer 20. Silicon carbide single crystalsubstrate 10 has a first main surface 11. Silicon carbide layer 20 is onfirst main surface 11. Silicon carbide layer 20 includes a second mainsurface 12 opposite to a surface 14 thereof in contact with siliconcarbide single crystal substrate 10. Second main surface 12 has amaximum diameter of more than or equal to 100 mm. Second main surface 12includes an outer peripheral region 125 which is within 3 mm from anouter edge 124 of second main surface 12, and a central region 126surrounded by outer peripheral region 125. Central region 126 has a hazeof less than or equal to 75 ppm.

The reliability of an insulating film is considered to be related to thesurface roughness of a silicon carbide epitaxial substrate on which theinsulating film is formed. As indexes for quantifying the degree of thesurface roughness, for example, an arithmetic average roughness (Ra), Saobtained by three-dimensionally expanding Ra, and the like are known.For example, Sa can be measured by observing a surface of a siliconcarbide epitaxial substrate with a white light interferometricmicroscope. The field of view for observation is 250 μm×250 μm, forexample. That is, Sa and Ra are roughnesses measured at a local regionin the surface of the silicon carbide epitaxial substrate, and thus theymay not represent the roughness of the entire surface. Accordingly, acharge-to-breakdown (Q_(BD)), which is used as one index for thereliability of an insulating film, and the surface roughness such as Raor Sa may not have a correlation therebetween.

Hence, the present inventors have focused on an index “haze” in order toevaluate the reliability of an insulating film. The haze is an indexindicating the degree of scattering in a surface. Specifically, lightsuch as a laser beam is emitted onto a surface of a silicon carbideepitaxial substrate, and scattered light of the light is observed. Thehaze is determined as a ratio of scattered light intensity to incidentlight intensity (unit: ppm). As a result of studies by the presentinventors, it has been found that the value of the haze has a strongcorrelation with charge-to-breakdown Q_(BD).

Furthermore, the present inventors have conducted a detailedinvestigation on the relation between the haze and charge-to-breakdownQ_(BD). As a result, it has been found that charge-to-breakdown Q_(BD)increases sharply when the haze becomes less than or equal to a certainvalue (specifically, less than or equal to 75 ppm). Ascharge-to-breakdown Q_(BD) is larger, the insulating film has a higherreliability. That is, the present inventors have found that thereliability of an insulating film formed on a surface of a siliconcarbide epitaxial substrate can be improved by setting the value of thehaze of the surface to less than or equal to 75 ppm.

(2) In silicon carbide epitaxial substrate 100 in accordance with (1)described above, second main surface 12 may be a (0001) plane, or aplane inclined from the (0001) plane by less than or equal to 8°.

Silicon carbide epitaxial substrate 100 in accordance with (2) describedabove has silicon carbide layer 20 formed on a Si plane side of siliconcarbide single crystal substrate 10. A carrier concentration in siliconcarbide layer 20 is calculated as the sum of nitrogen derived from a gassupplied as a dopant and nitrogen derived from other than the gas. Ofthe nitrogen captured into silicon carbide layer 20, the nitrogenderived from other than the gas supplied as a dopant is called abackground. The background is considered to be derived from residualnitrogen within a reaction chamber, for example.

In epitaxial growth on the Si plane side, the amount of change inbackground concentration with respect to the amount of change in C/Siratio is greater, when compared with epitaxial growth on a C plane side.Accordingly, in the epitaxial growth on the Si plane side, thebackground can be easily reduced by changing the C/Si ratio. On theother hand, in the epitaxial growth on the Si plane side, it isnecessary to enhance uniformity of the C/Si ratio in an in-planedirection in order to enhance uniformity of the background concentrationin the in-plane direction. Further, when an epitaxial layer is formed onthe Si plane side using conditions for a high C/Si ratio, there is atendency that the background concentration in the silicon carbide layeris reduced, whereas surface flatness of the silicon carbide layer isworsened. That is, in the epitaxial growth on the Si plane side, it hasbeen difficult to improve the surface flatness of the silicon carbidelayer while improving in-plane uniformity of the carrier concentrationin the silicon carbide layer.

According to silicon carbide epitaxial substrate 100 in accordance withthe present embodiment, in the epitaxial growth on the Si plane side,the surface flatness of the silicon carbide layer can be improved whileimproving the in-plane uniformity of the carrier concentration in thesilicon carbide layer, by using a manufacturing method described later.

(3) In silicon carbide epitaxial substrate 100 in accordance with (2)described above, in a direction parallel to second main surface 12, aratio of a standard deviation of the carrier concentration to an averagevalue of the carrier concentration in silicon carbide layer 20 may beless than or equal to 4%. The average value may be less than or equal to2×10¹⁶ cm⁻³.

According to silicon carbide epitaxial substrate 100 in accordance withthe present disclosure, the ratio of the standard deviation of thecarrier concentration to the average value of the carrier concentrationin a plane of silicon carbide layer 20 is less than or equal to 4%. Theratio is determined as a percentage of a value obtained by dividing thestandard deviation (σ) of the carrier concentration by the average value(ave) of the carrier concentration. Hereinafter, the ratio (σ/ave) maybe referred to as “in-plane uniformity”. The in-plane uniformityindicates that, as its value is smaller, the carrier concentration isdistributed more uniformly. It should be noted that the carrierconcentration in the present application means an effective carrierconcentration. For example, when the silicon carbide layer includeselectrons and holes, the effective carrier concentration is calculatedas an absolute value of a difference between electron density and holedensity. A method for measuring the carrier concentration will bedescribed later.

(4) In silicon carbide epitaxial substrate 100 in accordance with (2) or(3) described above, a groove portion 80 may be present in second mainsurface 12, groove portion 80 extending in one direction along secondmain surface 12, having a width in the one direction which is twice ormore a width thereof in a direction perpendicular to the one direction,and having a maximum depth from second main surface 12 of less than orequal to 10 nm.

(5) In the silicon carbide epitaxial substrate in accordance with (4)described above, groove portion 80 may include a first groove portion81, and a second groove portion 82 connected to first groove portion 81.First groove portion 81 may be at one end portion of groove portion 80in the one direction. Second groove portion 82 may extend from firstgroove portion 81 along the one direction to reach the other end portionopposite to the one end portion, and may have a depth from the secondmain surface which is smaller than a maximum depth of first grooveportion 81.

(6) In silicon carbide epitaxial substrate 100 in accordance with any of(2) to (5) described above, a pit 2 originating from a threading screwdislocation may be present in second main surface 12. Pit 2 may have anarea density of less than or equal to 1000 cm⁻². Within pit 2, a maximumdepth thereof from second main surface 12 may be more than or equal to 8nm.

(7) In silicon carbide epitaxial substrate 100 in accordance with (6)described above, pit 2 may have an area density of less than or equal to100 cm⁻².

(8) In silicon carbide epitaxial substrate 100 in accordance with (6)described above, pit 2 may have an area density of less than or equal to10 cm⁻².

(9) In silicon carbide epitaxial substrate 100 in accordance with (6)described above, pit 2 may have an area density of less than or equal to1 cm⁻².

(10) In silicon carbide epitaxial substrate 100 in accordance with anyof (6) to (9) described above, within pit 2, a maximum depth thereoffrom second main surface 12 may be more than or equal to 20 nm.

(11) In silicon carbide epitaxial substrate 100 in accordance with anyof (6) to (10) described above, pit 2 may have a planar shape includinga first width 51 extending in a first direction, and a second width 52extending in a second direction perpendicular to the first direction.First width 51 may be twice or more second width 52.

(12) In silicon carbide epitaxial substrate 100 in accordance with (1)described above, second main surface 12 may be a (000-1) plane, or aplane inclined from the (000-1) plane by less than or equal to 8°.

Silicon carbide epitaxial substrate 100 in accordance with (12)described above has silicon carbide layer 20 formed on the C plane sideof silicon carbide single crystal substrate 10. In silicon carbide layer20 formed on the C plane side, for example, improvement in channelmobility can be expected, when compared with silicon carbide layer 20formed on the Si plane side. However, on the C plane side of siliconcarbide single crystal substrate 10, it has been difficult to improvesurface flatness while improving the in-plane uniformity of the carrierconcentration, due to the reason described below.

As described above, in the epitaxial growth on the Si plane side, theamount of change in background concentration with respect to the amountof change in C/Si ratio is greater, when compared with the epitaxialgrowth on the C plane side. Specifically, in the epitaxial growth on theSi plane side, the background concentration can be reduced by about twoorders of magnitude by changing the C/Si ratio in a range of 0.5 to 2.However, in the epitaxial growth on the C plane side, even when the C/Siratio is changed in a similar manner, the change in the backgroundconcentration is less than one order of magnitude. Thus, in theepitaxial growth on the C plane side, it is difficult to reduce thebackground by the same technique as that for the Si plane side.Accordingly, in order to improve the in-plane uniformity of the carrierconcentration, it is necessary to sufficiently reduce residual nitrogenwhich can be the background.

The residual nitrogen is considered to be derived from nitrogen adsorbedto a peripheral member arranged around a silicon carbide single crystalsubstrate within a reaction chamber of a film forming device. Therefore,the residual nitrogen is considered to have a greater influence on anouter peripheral portion of the silicon carbide single crystalsubstrate, than a central portion thereof. The residual nitrogen can bereduced by so-called baking, for example. Desorption of the nitrogenadsorbed to the peripheral member can be promoted, for example, byincreasing the temperature within the reaction chamber and decreasingthe pressure within the reaction chamber during growth. Thereby, anabsolute value of a residual nitrogen concentration can be reduced. Onthe other hand, it becomes difficult to keep a uniform temperaturedistribution in a plane of the silicon carbide single crystal substrate.When the temperature distribution becomes nonuniform, the C/Si ratio inthe plane of the silicon carbide single crystal substrate also becomesnonuniform. As a result, it is considered that the in-plane uniformityof the carrier concentration in the silicon carbide layer and thesurface flatness of the silicon carbide layer are worsened.

From the above consideration, it is considered effective to uniformlydistribute the C/Si ratio in the plane of the silicon carbide singlecrystal substrate, in order to improve the in-plane uniformity of thecarrier concentration in the silicon carbide layer and to improve thesurface flatness of the silicon carbide layer, in the epitaxial growthon the C plane side.

According to silicon carbide epitaxial substrate 100 in accordance withthe present embodiment, also in the epitaxial growth on the C planeside, the surface flatness of the silicon carbide layer can be improvedwhile improving the in-plane uniformity of the carrier concentration inthe silicon carbide layer, by using the manufacturing method describedlater.

(13) In silicon carbide epitaxial substrate 100 in accordance with (12)described above, in the direction parallel to second main surface 12,the ratio of the standard deviation of the carrier concentration to theaverage value of the carrier concentration in silicon carbide layer 20may be less than or equal to 5%. The average value may be less than orequal to 2×10¹⁶ cm⁻³.

(14) In silicon carbide epitaxial substrate 100 in accordance with (13)described above, the ratio may be less than or equal to 3%.

(15) In silicon carbide epitaxial substrate 100 in accordance with (13)described above, the ratio may be less than or equal to 2%.

(16) In silicon carbide epitaxial substrate 100 in accordance with (13)described above, the ratio may be less than or equal to 1%.

(17) In silicon carbide epitaxial substrate 100 in accordance with anyof (12) to (16) described above, in second main surface 12, trapezoidaldefects 30, which are trapezoidal depressions, may have an area densityof less than or equal to 0.5 cm⁻², trapezoidal defects 30 may eachinclude an upper base portion 32 and a lower base portion 34intersecting with a <11-20> direction when viewed in plan view, upperbase portion 32 may have a width of more than or equal to 0.1 μm andless than or equal to 100 lower base portion 34 may have a width of morethan or equal to 50 μm and less than or equal to 5000 upper base portion32 may include a protruding portion 33, and lower base portion 34 mayinclude a plurality of step bunchings 35.

(18) In silicon carbide epitaxial substrate 100 in accordance with anyof (12) to (17) described above, in second main surface 12, triangulardefects 40 may have an area density of less than or equal to 0.5 cm⁻².

(19) In silicon carbide epitaxial substrate 100 in accordance with anyof (12) to (18) described above, silicon carbide epitaxial substrate 100may have a bow of less than or equal to 50 The “bow” is a value definedby “ASTM (American Society for Testing and Materials) F534”.

(20) In silicon carbide epitaxial substrate 100 in accordance with anyof (1) to (19) described above, the maximum diameter may be more than orequal to 150 mm.

(21) In silicon carbide epitaxial substrate 100 in accordance with anyof (1) to (20) described above, silicon carbide layer 20 may have athickness of more than or equal to 5 μm.

(22) A method for manufacturing a silicon carbide semiconductor device300 in accordance with the present disclosure includes the steps ofpreparing silicon carbide epitaxial substrate 100 in accordance with anyof (1) to (21) described above, and processing silicon carbide epitaxialsubstrate 100.

Details of Embodiment of Present Disclosure

Hereinafter, one embodiment of the present disclosure (hereinafter alsoreferred to as the “present embodiment”) will be described, although thepresent embodiment is not limited thereto.

(Silicon Carbide Epitaxial Substrate)

As shown in FIGS. 1 and 2, silicon carbide epitaxial substrate 100 inaccordance with the present embodiment has silicon carbide singlecrystal substrate 10 and silicon carbide layer 20. Silicon carbidesingle crystal substrate 10 includes first main surface 11, and a thirdmain surface 13 opposite to first main surface 11. Silicon carbide layer20 includes fourth main surface 14 in contact with silicon carbidesingle crystal substrate 10, and second main surface 12 opposite tofourth main surface 14. As shown in FIG. 1, silicon carbide epitaxialsubstrate 100 may have a first flat 5 extending in a first direction101. Silicon carbide epitaxial substrate 100 may have a second flat (notshown) extending in a second direction 102. First direction 101 is a<11-20> direction, for example. Second direction 102 is a <1-100>direction, for example.

Silicon carbide single crystal substrate 10 (hereinafter may be simplyreferred to as a “single crystal substrate”) is composed of a siliconcarbide single crystal. The polytype of the silicon carbide singlecrystal is 4H—SiC, for example. 4H-SiC is more excellent than otherpolytypes in terms of electron mobility, dielectric strength, and thelike. Silicon carbide single crystal substrate 10 contains an n typeimpurity such as nitrogen, for example. The conductivity type of siliconcarbide single crystal substrate 10 is n type, for example. First mainsurface 11 is, for example, a {0001} plane, or a plane inclined from the{0001} plane by less than or equal to 8°. When first main surface 11 isinclined from the {0001} plane, the inclination direction of a normal tofirst main surface 11 is the <11-20> direction, for example.

Silicon carbide layer 20 is an epitaxial layer formed on silicon carbidesingle crystal substrate 10. Silicon carbide layer 20 is on first mainsurface 11. Silicon carbide layer 20 is in contact with first mainsurface 11. Silicon carbide layer 20 contains an n type impurity such asnitrogen, for example. The conductivity type of silicon carbide layer 20is n type, for example. The concentration of the n type impuritycontained in silicon carbide layer 20 may be higher than theconcentration of the n type impurity contained in silicon carbide singlecrystal substrate 10. Silicon carbide layer 20 has a thickness of morethan or equal to 5 for example. The thickness of silicon carbide layer20 may be more than or equal to 10 more than or equal to 15 or more thanor equal to 20 The upper limit of the thickness of silicon carbide layer20 is not particularly limited. The upper limit of the thickness ofsilicon carbide layer 20 may be 150 for example.

As shown in FIG. 1, second main surface 12 has a maximum diameter 111 ofmore than or equal to 100 mm. Maximum diameter 111 may be more than orequal to 150 mm, more than or equal to 200 mm, or more than or equal to250 mm. The upper limit of maximum diameter 111 is not particularlylimited. The upper limit of maximum diameter 111 may be 300 mm, forexample.

Second main surface 12 may be, for example, a {0001} plane, or a planeinclined from the {0001} plane by less than or equal to 8°.Specifically, second main surface 12 may be a (0001) plane, or a planeinclined from the (0001) plane by less than or equal to 8°. Theinclination direction (off direction) of a normal to second main surface12 may be the <11-20> direction, for example. The inclination angle (offangle) from the {0001} plane may be more than or equal to 1°, or morethan or equal to 2°. The off angle may be less than or equal to 7°, orless than or equal to 6°.

As shown in FIG. 1, second main surface 12 includes outer peripheralregion 125, and central region 126 surrounded by outer peripheral region125. Outer peripheral region 125 is a region which is within 3 mm fromouter edge 124 of second main surface 12. In other words, in a radialdirection of second main surface 12, a distance 112 between outer edge124 and a boundary between outer peripheral region 125 and centralregion 126 is 3 mm.

(Haze)

Central region 126 has a haze of less than or equal to 75 ppm. The hazemay be less than or equal to 50 ppm, less than or equal to 25 ppm, orless than or equal to 20 ppm. The haze having a smaller value is morepreferable.

The haze is measured using SICA manufactured by Lasertec Corporation,for example. Specifically, a maximum haze value in rectangular regionsobtained by dividing one field of view for observation measuring 1.8mm±0.2 mm per side into 64 regions is derived. The one field of view forobservation includes an imaging region of 1024×1024 pixels. The maximumhaze value is obtained by calculating horizontal and vertical edgeintensity of the field of view for observation with a Sobel filter, andderiving an absolute value thereof. Through the above procedure, themaximum haze values in respective fields of view for observation areobserved in the entire surface of central region 126, which is a regionof second main surface 12 other than outer peripheral region 125. Anaverage value of the maximum haze values in the respective fields ofview for observation is defined as a haze value in central region 126.

(Bow)

Silicon carbide epitaxial substrate 100 is desirably a substrate havingless warpage. In other words, second main surface 12 is desirablysubstantially flat as shown in FIG. 2. Specifically, silicon carbideepitaxial substrate 100 may have a bow of less than or equal to 50 Thebow may be less than or equal to 40 less than or equal to 30 or lessthan or equal to 20 μm.

(In-Plane Uniformity of Carrier Concentration)

Silicon carbide layer 20 contains nitrogen, for example, as a dopant. Insilicon carbide layer 20, the average value of the carrier concentrationmay be less than or equal to 2×10¹⁶ cm⁻³. The average value of thecarrier concentration may be less than or equal to 1×10¹⁶ cm⁻³, lessthan or equal to 9×10¹⁵ cm⁻³, or less than or equal to 8×10¹⁵ cm⁻³.Further, the average value of the carrier concentration may be more thanor equal to 1×10¹⁵ cm⁻³, more than or equal to 5×10¹⁵ cm⁻³, or more thanor equal to 6×10¹⁵ cm⁻³, for example.

In the direction parallel to second main surface 12, the ratio (σ/ave)of the standard deviation of the carrier concentration to the averagevalue of the carrier concentration in silicon carbide layer 20 may beless than or equal to 4%. The ratio having a smaller value is morepreferable, and the ratio is ideally zero. The ratio may be less than orequal to 3%, less than or equal to 2%, or less than or equal to 1%.

Next, the method for measuring the carrier concentration will bedescribed. The carrier concentration is measured with a mercuryprobe-type C-V measurement device, for example. The probe has an area of0.01 cm², for example. Second main surface 12 includes an outerperipheral region 123 which is within 5 mm from outer edge 124, and acentral region 122 surrounded by outer peripheral region 123. Thecarrier concentration is measured in central region 122. In other words,the carrier concentration in outer peripheral region 123 is notmeasured. For example, in central region 122, positions obtained bysubstantially equally dividing a straight line which passes through thecenter of second main surface 12 and is parallel to first direction 101into 12 parts are defined as measurement positions. Similarly, positionsobtained by substantially equally dividing a straight line which passesthrough the center of second main surface 12 and is parallel to seconddirection 102 into 12 parts are defined as measurement positions. Theintersection of the two straight lines is defined as one of themeasurement positions. As shown in FIG. 3, the carrier concentration ismeasured at a total of 25 measurement positions in central region 122.An average value and a standard deviation of the carrier concentrationat the total of 25 measurement positions are calculated.

As shown in FIG. 2, silicon carbide layer 20 includes a surface layerregion 29 and a bottom layer region 26. Surface layer region 29 is aregion which extends from second main surface 12 toward fourth mainsurface 14 in a direction perpendicular to second main surface 12, andis within 10 μm from second main surface 12. The measurement depth isadjusted by an applied voltage. Bottom layer region 26 is a regionsandwiched between surface layer region 29 and a buffer layer 27. Thecarrier concentration is measured in surface layer region 29.Measurement data is plotted, with 1/C² being indicated on the axis ofordinates and V being indicated on the axis of abscissas. The carrierconcentration is estimated from the inclination of a straight line ofthe measurement data.

(Shallow Pit)

As shown in FIGS. 4 and 5, groove portion 80 may be present in secondmain surface 12. Groove portion 80 extends in one direction along secondmain surface 12 when viewed in a plan view of second main surface 12(i.e., a field of view viewed along the direction perpendicular tosecond main surface 12). More specifically, groove portion 80 extendsalong a step-flow growth direction 8 which is along the off direction ofthe off angle relative to the (0001) plane. That is, groove portion 80extends along a direction within a range of less than or equal to ±5°relative to the <11-20> direction, or along a direction within a rangeof less than or equal to ±5° relative to a <01-10> direction.

As shown in FIG. 5, groove portion 80 has a width 117 in the onedirection, which is twice or more, and preferably five times or more, awidth 119 thereof in a direction perpendicular to the one direction.Width 117 is more than or equal to 15 μm and less than or equal to 50and preferably more than or equal to 25 μm and less than or equal to 35μm. Width 119 is more than or equal to 1 μm and less than or equal to 5μm, and preferably more than or equal to 2 μm and less than or equal to3 μm.

As shown in FIG. 4, groove portion 80 extends from a threadingdislocation 25 present in silicon carbide layer 20, along step-flowgrowth direction 8 which is along the off direction of the off angle.More specifically, groove portion 80 includes first groove portion 81formed on threading dislocation 25, and second groove portion 82connected to first groove portion 81 and extending from first grooveportion 81 along step-flow growth direction 8.

First groove portion 81 is formed at one end portion (left end portionin FIG. 4) of groove portion 80 in step-flow growth direction 8.Further, first groove portion 81 has a maximum depth 114 from secondmain surface 12 of less than or equal to 10 nm. Maximum depth 114 is themaximum depth in the entire groove portion 80. First groove portion 81has a width 116, which is preferably less than or equal to 1 μm, andmore preferably less than or equal to 0.5 μm.

As shown in FIG. 4, second groove portion 82 extends from its portionconnected with first groove portion 81 to reach the other end portion(right end portion in FIG. 4) opposite to the one end portion. In otherwords, second groove portion 82 extends from first groove portion 81along one direction 8 to reach the other end portion opposite to the oneend portion. Second groove portion 82 has a depth 113 from second mainsurface 12 which is smaller than maximum depth 114 of first grooveportion 81. More specifically, second groove portion 82 extends alongstep-flow growth direction 8 while maintaining the depth shallower thanmaximum depth 114 of first groove portion 81. Depth 113 is preferablyless than or equal to 3 nm, more preferably less than or equal to 2 nm,and further preferably less than or equal to 1 nm. Further, secondgroove portion 82 has a width 118 which is more than or equal to 20 μm,for example, and is preferably more than or equal to 25 μm.

Groove portion 80 in second main surface 12 has an area density of morethan or equal to 10/mm², for example. The area density may be more thanor equal to 100/mm². The upper limit of the area density may be1000/mm².

As shown in FIGS. 4 and 6, a pit 90 may be provided in second mainsurface 12. As shown in FIG. 4, pit 90 originates from threadingdislocation 25 extending from silicon carbide single crystal substrate10 into silicon carbide layer 20. Pit 90 has a maximum depth 115, whichis more than 10 nm, and more specifically more than 20 nm. As shown inFIG. 6, pit 90 may have a triangular shape when viewed in plan view(i.e., when viewed from the direction perpendicular to second mainsurface 12).

(Deep Pit)

As shown in FIG. 7, a shallow pit 1 having a maximum depth of less than8 nm and a deep pit 2 having a maximum depth of more than or equal to 8nm may be present in second main surface 12. These pits may originatefrom a threading screw dislocation (TSD), a threading edge dislocation(TED), and the like in the epitaxial layer. Pit 2 is a groove-likemicroscopic defect. Pit 2 is considered to originate from a threadingscrew dislocation, a threading edge dislocation, and a threadingcomposite dislocation in silicon carbide layer 20. In the specificationof the present application, a threading composite dislocation containinga screw dislocation component is also regarded as a threading screwdislocation.

In second main surface 12, pit 2 originating from a threading screwdislocation and having a maximum depth 152 of more than or equal to 8 nmmay have an area density of less than or equal to 1000 cm⁻². The lowerthe area density of pit 2 is, the more it is desirable. The area densityof the pit may be less than or equal to 100 cm⁻², less than or equal to10 cm⁻², or less than or equal to 1 cm⁻². In second main surface 12, pit1 originating from a threading edge dislocation and having a maximumdepth of less than 8 nm may be present.

In second main surface 12, pit 2 originating from a threading screwdislocation and having maximum depth 152 of more than or equal to 20 nmmay have an area density of less than or equal to 1000 cm⁻². Pit 2having the maximum depth of more than or equal to 20 nm can also bedetected by shape definition in a defect inspection device describedlater. The area density of pit 2 originating from a threading screwdislocation and having the maximum depth of more than or equal to 20 nmmay be less than or equal to 100 cm⁻², less than or equal to 10 cm⁻², orless than or equal to 1 cm⁻².

FIGS. 8 to 10 are schematic views each showing an exemplary planar shapeof pit 2. The planar shape of pit 2 may have a circular shape such as acircular pit 60 shown in FIG. 8, a triangular shape such as a triangularpit 70 shown in FIG. 9, or a bar-like shape such as a bar-like pit 50shown in FIG. 10.

Bar-like pit 50 may have first width 51 extending in a third direction103, and second width 52 extending in a fourth direction 104perpendicular to third direction 103. First width 51 is twice or moresecond width 52. First width 51 may be five times or more second width52. First width 51 may be more than or equal to 5 μm, or more than orequal to 25 μm, for example. First width 51 may be less than or equal to50 μm, or less than or equal to 35 μm, for example. Second width 52 maybe more than or equal to 1 μm, or more than or equal to 2 μm, forexample. Second width 52 may be less than or equal to 5 μm, or less thanor equal to 4 μm, for example. Third direction 103 may be the <11-20>direction, or the <01-10> direction, for example.

(Method for Measuring Pit)

Whether or not a pit originates from a threading screw dislocation canbe confirmed by an etch pit method or an X-ray topography method. Whensilicon carbide layer 20 is formed on a (0001) plane side of siliconcarbide single crystal substrate 10, the etch pit method is used. Withthe etch pit method, a pit originating from a threading screwdislocation can be distinguished for example as described below. Itshould be noted that etching conditions shown herein are merely anexample, and the etching conditions may be changed depending on thethickness of the epitaxial layer, doping concentration, and the like,for example. The following conditions assume a case where the epitaxiallayer has a thickness of about 10 μm to 50 μm.

A potassium hydroxide (KOH) melt is used for etching. The temperature ofthe KOH melt is set to about 500 to 550° C. The etching time is set toabout 5 to 10 minutes. After the etching, second main surface 12 isobserved using a Nomarski differential interference microscope. A pitoriginating from a threading screw dislocation forms a larger etch pit,when compared with a pit originating from a threading edge dislocation.The etch pit originating from a threading screw dislocation has ahexagonal planar shape, for example, and a diagonal line of a hexagontypically has a length of about 30 to 50 μm. The etch pit originatingfrom a threading edge dislocation has a hexagonal planar shape, forexample, and is smaller than the etch pit originating from a threadingscrew dislocation. In the etch pit originating from a threading edgedislocation, a diagonal line of a hexagon typically has a length ofabout 15 to 20 μm.

When silicon carbide layer 20 is formed on a (000-1) plane side ofsilicon carbide single crystal substrate 10, the X-ray topography methodis used. When the silicon carbide layer has a thickness of about 10 μmto 50 μm, a diffraction vector g may be set as g=11-28, and apenetration length may be set to about 20 μm. A threading screwdislocation is observed with a stronger contrast, when compared with athreading edge dislocation.

The maximum depth from the main surface in the pit can be measured usingan AFM (Atomic Force Microscope). As the AFM, for example, “Dimension300” manufactured by Veeco or the like can be adopted. As a cantileverfor the AFM, “NCHV-10V” manufactured by Bruker or the like is suitable.Conditions for the AFM can be set as follows. The measurement mode isset to a tapping mode. The measurement region in the tapping mode is setto a square measuring 5 μm per side. For sampling in the tapping mode,the scanning speed within the measurement region is set to 5 seconds forone cycle, the number of scan lines is set to 512, and the number ofmeasurement points for each scan line is set to 512 points. Controlleddisplacement of the cantilever is set to 15.50 nm.

The shape of the “groove portion” can be specified by observing secondmain surface 12 using a defect inspection device including a confocaldifferential interference microscope. As the defect inspection deviceincluding a confocal differential interference microscope, WASAVI series“SICA 6X” manufactured by Lasertec Corporation or the like can be used.An objective lens is set to have a magnification of 10 times. Athreshold value of detection sensitivity of the defect inspection deviceis determined using the standard sample described above. Thereby, theshape of the “groove portion” formed in a measured sample can beevaluated quantitatively by using the defect inspection device.

The area density of the pit having a maximum depth from second mainsurface 12 of more than or equal to 8 nm is measured using AFMmeasurement and the defect inspection device together. The shape of apit having a maximum depth of more than or equal to 8 nm is defined byassociating depth data in the AFM measurement with a pit image inconfocal microscope measurement. The entire surface of second mainsurface 12 is analyzed to detect any pit which satisfies the definition.The area density of the pit can be calculated by dividing the number ofthe detected pits by a measurement area. It should be noted that,generally, the entire surface in this measurement does not include aregion which is not utilized for a semiconductor device. The regionwhich is not utilized for a semiconductor device is, for example, outerperipheral region 125 which is within 3 mm from outer edge 124 of secondmain surface 12.

(Film Forming Device)

Next, a configuration of a film forming device 200 used for a method formanufacturing silicon carbide epitaxial substrate 100 in accordance withthe present embodiment will be described.

As shown in FIG. 11, film forming device 200 is a hot wall-type CVD(Chemical Vapor Deposition) device, for example. Film forming device 200mainly has a heating element 220, a quartz tube 204, a heat insulator205, and an induction heating coil 206. A space surrounded by heatingelement 220 is a reaction chamber 201. Reaction chamber 201 is providedwith a susceptor plate 210 for holding silicon carbide single crystalsubstrate 10. Susceptor plate 210 is rotatable. Silicon carbide singlecrystal substrate 10 is placed on susceptor plate 210 with first mainsurface 11 facing upward.

Heating element 220 is made of graphite, for example. Induction heatingcoil 206 is wound around an outer periphery of quartz tube 204. Bysupplying a predetermined alternating current to induction heating coil206, heating element 220 is heated. Thereby, reaction chamber 201 isheated.

Film forming device 200 further has a gas inlet 207 and a gas outlet208. Gas outlet 208 is connected to an exhaust pump not shown. Arrows inFIG. 11 indicate flow of gas. A carrier gas, a source material gas, anda doping gas are introduced from gas inlet 207 into reaction chamber201, and are exhausted from gas outlet 208. The pressure within reactionchamber 201 is adjusted in accordance with a balance between the amountof the supplied gases and the amount of the exhausted gases.Hereinafter, matters to be taken into consideration in the manufacturingmethod of the present disclosure will be described.

(Arrangement of Susceptor Plate)

Generally, susceptor plate 210 and single crystal substrate 10 arearranged at substantially the center in an axial direction of reactionchamber 201. In the present disclosure, susceptor plate 210 and singlecrystal substrate 10 may be arranged on a downstream side, that is, on aside closer to gas outlet 208, relative to the center of reactionchamber 201, in order to sufficiently proceed a decomposition reactionof the source material gas until the source material gas reaches singlecrystal substrate 10. Thereby, it is expected that the C/Si ratio willbe uniformly distributed in the plane of the single crystal substrate.

In the present disclosure, a position at which the decompositionreaction of a Si source gas, of the source material gas, is predicted tobecome significant is referred to as a decomposition point 213 (see FIG.11). At decomposition point 213, the amount of a Si gas generated bythermal decomposition of the Si source gas increases sharply. Beyonddecomposition point 213, the amount of the Si gas gradually decreasestoward the downstream side. In contrast, the amount of a C gas generatedby thermal decomposition of a C source gas does not exhibit a maximumvalue and decreases monotonically around decomposition point 213.Therefore, the actual C/Si ratio in the plane of single crystalsubstrate 10 varies depending on the arrangement of single crystalsubstrate 10. The actual C/Si ratio used herein does not refer to a C/Siratio which is simply calculated from the flow rate of the Si source gasand the flow rate of the C source gas, but refers to a ratio of thenumber of C atoms contained in the C gas generated by thermaldecomposition to the number of Si atoms contained in the Si gasgenerated by thermal decomposition.

If a sufficient distance is not provided between decomposition point 213and single crystal substrate 10, there arises a significant differencein C/Si ratio between the outer peripheral portion of the single crystalsubstrate and the central portion of the single crystal substrate. Thus,it is conceivable that the amount of captured N varies in the plane ofsilicon carbide layer 20, and the in-plane uniformity of the carrierconcentration is deteriorated. As described above, it is desirable toprovide a sufficient distance between decomposition point 213 and singlecrystal substrate 10. A distance 153 (see FIG. 11) between decompositionpoint 213 and single crystal substrate 10 may be set to more than orequal to about 30 mm and less than or equal to about 150 mm, forexample.

Similarly, if ammonia gas is not sufficiently thermally decomposed on anupstream side of single crystal substrate 10, the amount of N generatedon single crystal substrate 10 by the thermal decomposition of theammonia gas varies. Accordingly, it is desirable to provide a sufficientdistance between a decomposition point for the ammonia gas and singlecrystal substrate 10. Thereby, the thermal decomposition of the ammoniagas can be promoted on the upstream side of single crystal substrate 10.This results in less variation in the amount of N on single crystalsubstrate 10, and can improve the in-plane uniformity of the carrierconcentration.

(Induction Heating Coil)

Generally, in film forming device 200 as shown in FIG. 10, inductionheating coil 206 is wound in the axial direction of the device with aconstant density of windings. The density of windings [the number ofwindings/m] is the number of windings of the coil per length in theaxial direction of the device. In the present disclosure, the density ofwindings of the induction heating coil may be changed in the axialdirection of the device. For example, a first region 221 adjacent to gasinlet 207, a third region 223 where single crystal substrate 10 isarranged, and a second region 222 located between first region 221 andthird region 223 may have respectively different densities of windings.For example, in order to bring decomposition point 213 closer to gasinlet 207, the density of windings in first region 221 may be set higherthan the density of windings in second region 222. Alternatively, inorder to uniformize temperature distribution in the plane of singlecrystal substrate 10, the density of windings in third region 223 may beset higher than the density of windings in second region 222.

(Method for Manufacturing Silicon Carbide Epitaxial Substrate)

Next, a method for manufacturing the silicon carbide epitaxial substratein accordance with the present embodiment will be described.

First, a silicon carbide single crystal having a polytype of 6H ismanufactured by a sublimation method, for example. Silicon carbidesingle crystal substrate 10 is prepared by slicing the silicon carbidesingle crystal using a wire saw, for example. Silicon carbide singlecrystal substrate 10 has first main surface 11, and third main surface13 opposite to first main surface 11. First main surface 11 is a planeinclined from the (0001) plane by less than or equal to 8°, for example.As shown in FIG. 11, silicon carbide single crystal substrate 10 isarranged in a recess of susceptor plate 210 such that first main surface11 is exposed from susceptor plate 210. Next, silicon carbide layer 20is formed on silicon carbide single crystal substrate 10 by theepitaxial growth, using film forming device 200.

FIG. 12 is a timing chart showing an example of condition control in theepitaxial growth of the present disclosure. A first time point (t1)indicates a time point at which single crystal substrate 10 is arrangedon susceptor plate 210. At the first time point (t1), the temperaturewithin reaction chamber 201 is close to room temperature, and thepressure within reaction chamber 201 is equal to the atmosphericpressure. From a second time point (t2), reduction of the pressurewithin reaction chamber 201 is started. At a third time point (t3), thepressure within reaction chamber 201 reaches a first pressure (P1). Thefirst pressure (P1) is about 1×10⁻⁶ Pa, for example.

From the third time point (t3), temperature rising is started. In thepresent disclosure, the temperature within reaction chamber 201 may beheld at a first temperature (T1) from a fourth time point (t4) to afifth time point (t5) during the temperature rising. The firsttemperature (T1) may be about 900 to 1300° C., for example. The holdingtime may be about 5 to 20 minutes, for example. Through this operation,reduction of deviation between the temperature of susceptor plate 210and the temperature of single crystal substrate 10, and uniformtemperature distribution in the plane of single crystal substrate 10 areexpected.

From the fifth time point (t5), the temperature rising is resumed. Inthe present disclosure, hydrogen (H₂) gas serving as the carrier gas maybe introduced from a sixth time point (t6) during the temperaturerising. A second temperature (T2) at the sixth time point (t6) may beabout 1300 to 1500° C., for example. The flow rate of the hydrogen gas(FH) may be about 50 to 200 slm, or about 100 to 150 slm, for example.The unit “slm” of the flow rate represents “L/min” under standardconditions (0° C., 101.3 kPa). Through this operation, for example,reduction of residual nitrogen within reaction chamber 201 is expected.

By the introduction of the hydrogen gas, the pressure within reactionchamber 201 changes from the first pressure (P1) to a second pressure(P2). The second pressure (P2) may be more than or equal to about 5 kPaand less than or equal to about 40 Pa, or more than or equal to about 5kPa and less than or equal to about 15 kPa, for example.

At a seventh time point (t7), the temperature within reaction chamber201 reaches a third temperature (T3). The third temperature (T3) is agrowth temperature at which the epitaxial growth proceeds. The thirdtemperature (T3) may be about 1500 to 1700° C., or about 1550 to 1650°C., for example.

From an eighth time point (t8), the source material gas and the dopinggas are introduced. In the present disclosure, ammonia (NH₃) gas is usedas the doping gas. By using the ammonia gas, improvement in in-planeuniformity can be expected. The ammonia gas may be thermally decomposedbeforehand at a stage before being introduced into reaction chamber 201.The doping gas may contain nitrogen (N₂) gas and the like, for example,in addition to the ammonia gas.

The source material gas includes the Si source gas and the C source gas.As the Si source gas, for example, silane (SiH₄) gas, disilane (Si₂H₆)gas, dichlorosilane (SiH₂Cl₂) gas, trichlorosilane (SiHCl₃) gas, silicontetrachloride (SiCl₄) gas, or the like can be used. As the C source gas,for example, methane (CH₄) gas, ethane (C₂H₆) gas, propane (C₃H₈) gas,acetylene (C₂H₂) gas, or the like can be used.

From the eighth time point (t8) to a ninth time point (t9), siliconcarbide layer 20 is formed on silicon carbide single crystal substrate10 by the epitaxial growth. Susceptor plate 210 is rotating whilesilicon carbide layer 20 is being formed by the epitaxial growth.Silicon carbide layer 20 includes buffer layer 27, and a drift layer 28formed on buffer layer 27 (see FIG. 2).

As shown in FIG. 13, from the eighth time point (t8) to a time point(t81), buffer layer 27 is formed. In the step of forming buffer layer27, the temperature (T3) within reaction chamber 201 is 1630° C., forexample. The number of rotations (R1) of susceptor plate 210 is 60 rpm,for example. The pressure (P2) within reaction chamber 201 is 8 kPa. Theflow rate of the silane gas (FS1) is 46 sccm, and the flow rate of thepropane gas (FC1) is 14 sccm. The volume ratio of silane to hydrogen is0.04%, for example. The C/Si ratio (A1) of the source material gas is0.9, for example. From the eighth time point (t8) to the time point(t81), it takes more than or equal to about 5 minutes and less than orequal to about 10 minutes, for example.

Next, from the time point (t81) to a time point (t83), a switching stepis performed. Specifically, from the time point (t81) to a time point(t82), the number of rotations of susceptor plate 210 decreases from thefirst number of rotations (R1) to a second number of rotations (R2). Thefirst number of rotations (R1) is 60 rpm, for example. The second numberof rotations (R2) is less than 10 rpm, for example. From the time point(t81) to the time point (t83), the temperature (T3) within reactionchamber 201 is 1630° C., for example, the pressure (P2) within reactionchamber 201 is 8 kPa, the flow rate of the silane gas (FS1) is 46 sccmand the flow rate of the propane gas (FC1) is 14 sccm, the volume ratioof silane to hydrogen is 0.04%, for example, and the C/Si ratio (A1) ofthe source material gas is 0.9, for example. From the time point (t82)to the time point (t83), the number of rotations of susceptor plate 210may be maintained at the second number of rotations (R2).

Next, from the time point (t83) to the ninth time point (t9), driftlayer 28 is formed on buffer layer 27. Specifically, from the time point(t83) to a time point (t84), the number of rotations of susceptor plate210 increases from the second number of rotations (R2) to the firstnumber of rotations (R1). While the number of rotations of susceptorplate 210 is increasing, the temperature (T3) within reaction chamber201 is 1630° C., for example, and the pressure (P2) within reactionchamber 201 is 8 kPa. From the time point (t83) to the time point (t84),the flow rate of the silane gas increases from the first flow rate (FS1)to a second flow rate (FS2). The first flow rate (FS1) is 46 sccm, forexample. The second flow rate (FS2) is 92 sccm, for example. From thetime point (t83) to the time point (t84), the flow rate of the propanegas increases from the first flow rate (FC1) to a second flow rate(FC2). The first flow rate (FC1) is 14 sccm, for example. The secondflow rate (FC2) is 30 sccm, for example. The C/Si ratio of the sourcematerial gas increases from the first ratio (A1) to a second ratio (A2).The first ratio (A1) is 0.9, for example. The second ratio (A2) is 1.0,for example. From the time point (t83) to the time point (t84), it takesabout 3 minutes to 30 minutes, for example.

From the time point (t83) to the time point (t84), the flow rate of thesilane gas may once decrease from the first flow rate (FS1) to a flowrate lower than the first flow rate (FS1), and then increase to thesecond flow rate (FS2). Similarly, from the time point (t83) to the timepoint (t84), the flow rate of the propane gas may once decrease from thefirst flow rate (FC1) to a flow rate lower than the first flow rate(FC1), and then increase to the second flow rate (FC2).

From the time point (t84) to the ninth time point (t9), the temperature(T3) within reaction chamber 201 is 1630° C., for example. The number ofrotations (R1) of susceptor plate 210 is 60 rpm, for example. Thepressure (P2) within reaction chamber 201 is 8 kPa. The flow rate of thesilane gas (FS2) is 92 sccm, and the flow rate of the propane gas (FC2)is 30 sccm. The volume ratio of silane to hydrogen is 0.08%, forexample. The C/Si ratio (A2) of the source material gas is 1.0, forexample. From the time point (t84) to the ninth time point (t9), ittakes about one hour, for example.

As shown in FIG. 12, at the ninth time point (t9), supply of the sourcematerial gas is stopped, and reduction of temperature within reactionchamber 201 is started. After the temperature of reaction chamber 201 isreduced to be close to the room temperature, at a tenth time point(t10), reaction chamber 201 is opened to the atmosphere. At an eleventhtime point (t11), silicon carbide epitaxial substrate 100 is removedfrom film forming device 200.

It should be noted that the following steps may be performed in the stepof forming the drift layer. Thereby, the effect of suppressing formationof a pit is expected.

As shown in FIGS. 4 and 7, drift layer 28 may include a first layer 23and a second layer 24. The step of forming drift layer 28 may includethe step of forming first layer 23, the step of reconstructing a surfaceof first layer 23, and the step of forming second layer 24.

The source material gas in the step of forming the first layer may be amixed gas of silane gas and propane gas, for example. In the step offorming the first layer, the C/Si ratio of the source material gas isadjusted to less than 1. The C/Si ratio may be more than or equal to0.5, more than or equal to 0.6, or more than or equal to 0.7, forexample, as long as the C/Si ratio is less than 1. Further, the C/Siratio may be less than or equal to 0.95, less than or equal to 0.9, orless than or equal to 0.8, for example. The flow rate of the silane gasand the flow rate of the propane gas may be adjusted as appropriate in arange of about 10 to 100 sccm, for example, to achieve a desired C/Siratio.

The film formation rate in the step of forming the first layer may bemore than or equal to about 3 μm/h and less than or equal to about 30μm/h, for example. The first layer has a thickness of more than or equalto 0.1 μm and less than or equal to 150 for example. The thickness ofthe first layer may be more than or equal to 0.2 more than or equal to 1μm, more than or equal to 10 μm, or more than or equal to 15 μm.Further, the thickness of the first layer may be less than or equal to100 μm, less than or equal to 75 μm, or less than or equal to 50 μm.

Next, the step of reconstructing the surface of the first layer isperformed. The step of reconstructing the surface may be performedcontinuously after the step of forming the first layer. Alternatively, apredetermined halt time may be provided between the step of forming thefirst layer and the step of reconstructing the surface. In the step ofreconstructing the surface, the temperature of the susceptor may beincreased by about 10 to 30° C.

In the step of reconstructing the surface, a mixed gas including asource material gas having a C/Si ratio of less than 1 and hydrogen gasis used. The C/Si ratio of the source material gas may be lower than theC/Si ratio in the step of forming the first layer. The C/Si ratio may bemore than or equal to 0.5, more than or equal to 0.6, or more than orequal to 0.7, as long as the C/Si ratio is less than 1. Further, theC/Si ratio may be less than or equal to 0.95, less than or equal to 0.9,or less than or equal to 0.8, for example.

In the step of reconstructing the surface, there may be used a sourcematerial gas different from the source material gas used in the step offorming the first layer and the step of forming the second layerdescribed later. In this way, the effect of suppressing formation of apit is expected to be enhanced. For example, there is conceivable aconfiguration such that, in the step of forming the first layer and thestep of forming the second layer described later, silane gas and propanegas are used, whereas in the step of reconstructing the surface,dichlorosilane and acetylene are used.

In the step of reconstructing the surface, the ratio of the flow rate ofthe source material gas to the flow rate of the hydrogen gas may bedecreased, when compared with the step of forming the first layer andthe step of forming the second layer described later. Thereby, theeffect of suppressing formation of a deep pit is expected to beenhanced.

The flow rate of the hydrogen gas in the mixed gas may be more than orequal to about 100 slm and less than or equal to about 150 slm, forexample. The flow rate of the hydrogen gas may be about 120 slm, forexample. The flow rate of the Si source gas in the mixed gas may be morethan or equal to 1 sccm and less than or equal to 5 sccm, for example.The lower limit of the flow rate of the Si source gas may be 2 sccm. Theupper limit of the flow rate of the Si source gas may be 4 sccm. Theflow rate of the C source gas in the mixed gas may be more than or equalto 0.3 sccm and less than or equal to 1.6 sccm, for example. The lowerlimit of the flow rate of the C source gas may be 0.5 sccm or 0.7 sccm.The upper limit of the flow rate of the C source gas may be 1.4 sccm or1.2 sccm.

In the step of reconstructing the surface, it is desirable to adjustvarious conditions such that etching by the hydrogen gas is comparableto epitaxial growth by the source material gas. For example, it isconceivable to adjust the flow rate of the hydrogen gas and the flowrate of the source material gas to attain a film formation rate of about0±0.5 μm/h. The film formation rate may be adjusted to about 0±0.4 μm/h,may be adjusted to about 0±0.3 μm/h, may be adjusted to about 0±0.2μm/h, or may be adjusted to about 0±0.1 μm/h. Thereby, the effect ofsuppressing formation of a pit is expected to be enhanced.

The treatment time in the step of reconstructing the surface is morethan or equal to about 30 minutes and less than or equal to about 10hours, for example. The treatment time may be less than or equal to 8hours, less than or equal to 6 hours, less than or equal to 4 hours, orless than or equal to 2 hours.

After the surface of the first layer is reconstructed, the step offorming the second layer on this surface is performed. Second layer 24(see FIGS. 4 and 7) is formed using a source material gas having a C/Siratio of more than or equal to 1. The C/Si ratio may be more than orequal to 1.05, more than or equal to 1.1, more than or equal to 1.2,more than or equal to 1.3, or more than or equal to 1.4, for example, aslong as the C/Si ratio is more than or equal to than 1. Further, theC/Si ratio may be less than or equal to 2.0, less than or equal to 1.8,or less than or equal to 1.6.

The source material gas in the step of forming the second layer may bethe same as or different from the source material gas used in the stepof forming the first layer. The source material gas may be silane gasand propane gas, for example. The flow rate of the silane gas and theflow rate of the propane gas may be adjusted as appropriate in a rangeof about 10 to 100 sccm, for example, to achieve a desired C/Si ratio.The flow rate of the carrier gas may be about 50 slm to 200 slm, forexample.

The film formation rate in the step of forming the second layer may bemore than or equal to about 5 μm/h and less than or equal to about 100μm/h, for example. The second layer has a thickness of more than orequal to 1 μm and less than or equal to 150 μm, for example. Thethickness of the second layer may be more than or equal to 5 μm, morethan or equal to 10 μm, or more than or equal to 15 μm. Further, thethickness of the second layer may be less than or equal to 100 μm, lessthan or equal to 75 μm, or less than or equal to 50 μm.

The thickness of the second layer may be the same as or different fromthe thickness of the first layer. The second layer may be thinner thanthe first layer. For example, the ratio of the thickness of the secondlayer to the thickness of the first layer may be more than or equal toabout 0.01 and less than or equal to about 0.9. Here, the ratio of thethicknesses represents a value obtained by dividing the thickness of thesecond layer by the thickness of the first layer having been through thestep of reconstructing the surface. The ratio of the thicknesses may beless than or equal to 0.8, less than or equal to 0.7, less than or equalto 0.6, less than or equal to 0.5, less than or equal to 0.4, less thanor equal to 0.3, less than or equal to 0.2, or less than or equal to0.1. Thereby, the effect of suppressing formation of a pit is expectedto be enhanced.

Next, a method for further improving the in-plane uniformity of thecarrier concentration will be described.

As shown in FIG. 14, second arrows 92 indicate a direction in whichsusceptor plate 210 rotates. Further, first arrows 91 indicate adirection in which the source material gas flows. The source materialgas includes a dopant gas. As indicated by first arrows 91, the sourcematerial gas flows along one direction. However, since susceptor plate210 rotates, supply of the source material gas to silicon carbide singlecrystal substrate 10 becomes substantially uniform in the direction inwhich susceptor plate 210 rotates.

Desirably, susceptor plate 210 and heating element 220 are composed of amaterial having a low nitrogen concentration, in order to reduce thebackground concentration of nitrogen in silicon carbide layer 20. InFIG. 14, a third arrow 93 indicates nitrogen emitted from susceptorplate 210, and a fourth arrow 94 indicates nitrogen emitted from heatingelement 220. When susceptor plate 210 and heating element 220 containnitrogen, the nitrogen is supplied to silicon carbide single crystalsubstrate 10 and silicon carbide layer 20 together with the sourcematerial gas as indicated by third arrow 93 and fourth arrow 94, andserves as the background of nitrogen.

Due to the influence of the background, the in-plane uniformity of thecarrier concentration (nitrogen concentration) is reduced. Such atendency is significant in a case where the nitrogen concentration insilicon carbide layer 20 is set to a low concentration. The case wherethe nitrogen concentration is set to a low concentration is a case wherethe nitrogen concentration is set to less than or equal to 2×10¹⁶ cm⁻³,for example.

Accordingly, the present embodiment adopts a configuration in whichnitrogen contained in susceptor plate 210 and heating element 220 has alow concentration. FIG. 15 is a schematic cross sectional view showing aconfiguration in the vicinity of susceptor plate 210. As shown in FIG.15, susceptor plate 210 includes a first base member 211, and a firstcoat portion 212 covering first base member 211. In addition, heatingelement 220 includes a second base member 225, and a second coat portion226 covering second base member 225.

First base member 211 and second base member 225 are composed of acarbon material, for example. The nitrogen concentration in first basemember 211 and second base member 225 is preferably less than or equalto 10 ppm, and more preferably less than or equal to 5 ppm. First coatportion 212 and second coat portion 226 are composed of silicon carbide(SiC), tantalum carbide (TaC), or the like, for example. The nitrogenconcentration in first coat portion 212 and second coat portion 226 ispreferably less than or equal to 10 ppm, and more preferably less thanor equal to 5 ppm. The arithmetic average roughness (Ra) of a surface offirst coat portion 212 may be less than or equal to the arithmeticaverage roughness (Ra) of third main surface 13 of single crystalsubstrate 10 which is to come into contact with first coat portion 212.Thereby, uniform temperature distribution in the plane of the singlecrystal substrate is expected.

In FIG. 15, fifth arrows 95 indicate nitrogen emitted from first basemember 211, and sixth arrows 96 indicate nitrogen emitted from firstcoat portion 212. In addition, seventh arrows 97 indicate nitrogenemitted from second base member 225, and eighth arrows 98 indicatenitrogen emitted from second coat portion 226. These nitrogens can besufficiently reduced by setting the nitrogen concentration in eachmember to a low concentration as described above. Thereby, thebackground concentration of nitrogen in silicon carbide layer 20 can beset to less than or equal to 1×10¹⁵ cm⁻³.

(Variation of Silicon Carbide Epitaxial Substrate)

Next, a configuration of a silicon carbide epitaxial substrate inaccordance with a variation of the present embodiment will be described.As shown in FIG. 1, second main surface 12 of silicon carbide epitaxialsubstrate 100 in accordance with the variation may be a (000-1) plane,or a plane inclined from the (000-1) plane by less than or equal to 8°.The inclination direction (off direction) of a normal to second mainsurface 12 may be the <11-20> direction, for example. The inclinationangle (off angle) from the (000-1) plane may be more than or equal to1°, more than or equal to 2°, or more than or equal to 3°. The off anglemay be less than or equal to 7°, or less than or equal to 6°.

In silicon carbide layer 20, the average value of the carrierconcentration is less than or equal to 2×10¹⁶ cm⁻³. The average value ofthe carrier concentration may be less than or equal to 1×10¹⁶ cm⁻³, lessthan or equal to 9×10¹⁵ cm⁻³, or less than or equal to 8×10¹⁵ cm⁻³.Further, the average value of the carrier concentration may be more thanor equal to 1×10¹⁵ cm⁻³, more than or equal to 5×10¹⁵ cm⁻³, or more thanor equal to 6×10¹⁵ cm⁻³, for example.

In the direction parallel to second main surface 12, the ratio (σ/ave)of the standard deviation of the carrier concentration to the averagevalue of the carrier concentration in silicon carbide layer 20 may beless than or equal to 5%. The ratio having a smaller value is morepreferable, and the ratio is ideally zero. The ratio may be less than orequal to 4%, less than or equal to 3%, less than or equal to 2%, or lessthan or equal to 1%.

As shown in FIG. 3, second main surface 12 includes outer peripheralregion 123, an intermediate region 127, and a central portion 121. Aregion composed of outer peripheral region 123 and intermediate region127 is a region which is within 30 mm from outer edge 124 of second mainsurface 12 toward the center of second main surface 12.

(Trapezoidal Defects)

According to the present disclosure, the defect density of trapezoidaldefects in second main surface 12 may be able to be reduced. That is, inthe present disclosure, trapezoidal defects in second main surface 12may have a defect density of less than or equal to 0.5 cm⁻². The lowerthe defect density of trapezoidal defects is, the more it is preferable,and the defect density of trapezoidal defects is ideally zero. Thedefect density of trapezoidal defects may be less than or equal to 0.3cm⁻², less than or equal to 0.1 cm⁻², or less than or equal to 0.01cm⁻².

A trapezoidal defect is a trapezoidal depression in second main surface12. As shown in FIG. 16, trapezoidal defect 30 includes upper baseportion 32 and lower base portion 34 intersecting with the <11-20>direction. Upper base portion 32 has a width 155 of more than or equalto 0.1 μm and less than or equal to 100 μm. Lower base portion 34 has awidth 156 of more than or equal to 50 μm and less than or equal to 5000μm.

FIG. 17 is a schematic cross sectional view taken along a line XVII-XVIIin FIG. 16. As shown in FIG. 17, upper base portion 32 may includeprotruding portion 33. Protruding portion 33 may be locatedsubstantially at the center of upper base portion 32. In upper baseportion 32, protruding portion 33 protrudes relative to a portion otherthan protruding portion 33. Protruding portion 33 has a height 157 ofmore than or equal to about 5 nm and less than or equal to about 20 nm.Height 157 of protruding portion 33 can be measured with a white lightinterferometric microscope (such as “BW-D507” manufactured by NikonCorporation), for example. A mercury lamp can be adopted as a lightsource for the white light interferometric microscope. The field of viewfor observation can be set to 250 μm×250 μm.

FIG. 18 is a schematic cross sectional view taken along a lineXVIII-XVIII in FIG. 16. An angle θ in FIG. 18 corresponds to an offangle. Inside trapezoidal defect 30, that is, in a region between upperbase portion 32 and lower base portion 34, the surface of siliconcarbide layer 20 slightly recedes toward single crystal substrate 10. Inother words, trapezoidal defect 30 includes a recess formed in secondmain surface 12. Trapezoidal defect 30 may have an origin 31 at aninterface between single crystal substrate 10 and silicon carbide layer20. As shown in FIG. 17, a dislocation extending from origin 31 may beconnected with protruding portion 33 described above.

FIG. 19 is an enlarged view of a region XIX in FIG. 16. As shown in FIG.19, lower base portion 34 may include the plurality of step bunchings35. The “step bunching” refers to a linear defect in which a pluralityof atomic steps form a bunch and produce a level difference of more thanor equal to 1 nm. The size of the level difference in the step bunchingmay be about 1 to 5 nm, for example. The size of the level difference inthe step bunching can be measured with an AFM, for example. The numberof the step bunchings included in lower base portion 34 may be about 2to 100, or about 2 to 50, for example. The number of the step bunchingsincluded in lower base portion 34 can also be counted by observing lowerbase portion 34 with the AFM.

As the AFM, for example, “Dimension 300” manufactured by Veeco or thelike can be adopted. As a cantilever for the AFM, “NCHV-10V”manufactured by Bruker or the like is suitable. Conditions for the AFMare set as follows. The measurement mode is set to a tapping mode. Themeasurement region in the tapping mode is set to a square measuring 20μm per side. The measurement depth is set to 1.0 μm. For sampling in thetapping mode, the scanning speed within the measurement region is set to5 seconds for one cycle, the number of scan lines is set to 512, and thenumber of measurement points for each scan line is set to 512 points.Controlled displacement of the cantilever is set to 15.50 nm.

(Triangular Defects)

According to the present disclosure, the defect density of triangulardefects in second main surface 12 may be able to be reduced. That is, inthe present disclosure, triangular defects in second main surface 12 mayhave a density of less than or equal to 0.5 cm⁻². As shown in FIG. 20,triangular defect 40 is a triangular depression in second main surface12. Triangular defect 40 includes a side intersecting with the <11-20>direction. Each side has a length of about 1 to 1000 μm. The lower thedefect density of triangular defects is, the more it is preferable, andthe defect density of triangular defects is ideally zero. The defectdensity of triangular defects may be less than or equal to 0.3 cm⁻²,less than or equal to 0.1 cm⁻², or less than or equal to 0.01 cm⁻².

(Method for Measuring Defect Density)

The trapezoidal defects and the triangular defects in second mainsurface 12 can be observed using a Nomarski-type optical microscope (forexample, “MX-51” manufactured by Olympus Corporation), for example. Thedefect densities of the trapezoidal defects and the triangular defectscan be calculated, for example, by analyzing the entire surface ofsecond main surface 12 at a magnification of 50 times to 400 times, anddividing the number of each type of the detected defects by the area ofsecond main surface 12. It should be noted that, generally, the entiresurface described above does not include a region which is not utilizedfor a semiconductor device. The region which is not utilized for asemiconductor device is, for example, a region which is 3 mm from anedge of a substrate.

Next, a method for manufacturing the silicon carbide epitaxial substratein accordance with the variation will be described. In the method formanufacturing the silicon carbide epitaxial substrate in accordance withthe variation, silicon carbide single crystal substrate 10 is arrangedon susceptor plate 210 such that second main surface 12 of siliconcarbide single crystal substrate 10 faces upward (see FIG. 11). Secondmain surface 12 is a (000-1) plane, or a plane inclined from the (000-1)plane by less than or equal to 8°. Other steps are the same as those inthe manufacturing method in accordance with the embodiment, and thusthey are not repeated.

(Method for Manufacturing Silicon Carbide Semiconductor Device)

Next, a method for manufacturing silicon carbide semiconductor device300 in accordance with the present embodiment will be described.

The method for manufacturing the silicon carbide semiconductor device inaccordance with the present embodiment mainly has an epitaxial substratepreparing step (S10: FIG. 21) and a substrate processing step (S20: FIG.21).

First, the silicon carbide epitaxial substrate preparing step (S10: FIG.21) is performed. Specifically, a silicon carbide epitaxial substrate isprepared by the method for manufacturing the silicon carbide epitaxialsubstrate described above.

Next, the substrate processing step (S20: FIG. 21) is performed.Specifically, a silicon carbide semiconductor device is manufactured byprocessing the silicon carbide epitaxial substrate. The “processing”includes various types of processing such as ion implantation, heattreatment, etching, oxide film formation, electrode formation, dicing,and the like, for example. That is, the substrate processing step mayinclude at least any one of ion implantation, heat treatment, etching,oxide film formation, electrode formation, and dicing.

Hereinafter, a method for manufacturing a MOSFET (Metal OxideSemiconductor Field Effect Transistor) as an exemplary silicon carbidesemiconductor device will be described. The substrate processing step(S20: FIG. 21) includes an ion implantation step (S21: FIG. 21), anoxide film forming step (S22: FIG. 21), an electrode forming step (S23:FIG. 21), and a dicing step (S24: FIG. 21).

First, the ion implantation step (S21: FIG. 21) is performed. A p typeimpurity such as aluminum (A1), for example, is implanted into secondmain surface 12 on which a mask (not shown) having an opening is formed.Thereby, a body region 132 having a p type conductivity type is formed.Next, an n type impurity such as phosphorus (P), for example, isimplanted into a predetermined position within body region 132. Thereby,a source region 133 having an n type conductivity type is formed. Next,a p type impurity such as aluminum is implanted into a predeterminedposition within source region 133. Thereby, a contact region 134 havingthe p type conductivity type is formed (see FIG. 22).

In silicon carbide layer 20, a portion other than body region 132,source region 133, and contact region 134 serves as a drift region 131.Source region 133 is separated from drift region 131 by body region 132.Ion implantation may be performed with silicon carbide epitaxialsubstrate 100 being heated to more than or equal to about 300° C. andless than or equal to about 600° C. After the ion implantation,activation annealing is performed on silicon carbide epitaxial substrate100. By the activation annealing, the impurities implanted into siliconcarbide layer 20 are activated, and carriers are generated in eachregion. The atmosphere for the activation annealing may be an argon (Ar)atmosphere, for example. The temperature for the activation annealingmay be about 1800° C., for example. The time for the activationannealing may be about 30 minutes, for example.

Next, the oxide film forming step (S22: FIG. 21) is performed. An oxidefilm 136 is formed on second main surface 12 (see FIG. 23) by heatingsilicon carbide epitaxial substrate 100 in an atmosphere containingoxygen, for example. Oxide film 136 is composed of silicon dioxide(SiO₂) or the like, for example. Oxide film 136 functions as a gateinsulating film. The temperature for thermal oxidation treatment may beabout 1300° C., for example. The time for the thermal oxidationtreatment may be about 30 minutes, for example.

After oxide film 136 is formed, heat treatment may be further performedin a nitrogen atmosphere. For example, the heat treatment may beperformed in an atmosphere of nitric oxide (NO), nitrous oxide (N₂O), orthe like, at about 1100° C., for about one hour. Thereafter, heattreatment may be further performed in an argon atmosphere. For example,the heat treatment may be performed in an argon atmosphere, at about1100 to 1500° C., for about one hour.

Next, the electrode forming step (S23: FIG. 21) is performed. A firstelectrode 141 is formed on oxide film 136. First electrode 141 functionsas a gate electrode. First electrode 141 is formed by a CVD method, forexample. First electrode 141 is composed of polysilicon or the likecontaining an impurity and having electrical conductivity, for example.First electrode 141 is formed at a position facing source region 133 andbody region 132.

Next, an interlayer insulating film 137 covering first electrode 141 isformed. Interlayer insulating film 137 is formed by the CVD method, forexample. Interlayer insulating film 137 is composed of silicon dioxideor the like, for example. Interlayer insulating film 137 is formed tocome into contact with first electrode 141 and oxide film 136. Next,oxide film 136 and interlayer insulating film 137 at a predeterminedposition are removed by etching. Thereby, source region 133 and contactregion 134 are exposed from oxide film 136.

At the exposed portion, a second electrode 142 is formed by a sputteringmethod, for example. Second electrode 142 functions as a sourceelectrode. Second electrode 142 is composed of titanium, aluminum,silicon, or the like, for example. After second electrode 142 is formed,second electrode 142 and silicon carbide epitaxial substrate 100 areheated at a temperature of about 900 to 1100° C., for example. Thereby,second electrode 142 and silicon carbide epitaxial substrate 100 comeinto ohmic contact with each other. Next, an interconnection layer 138is formed to come into contact with second electrode 142.Interconnection layer 138 is composed of a material containing aluminum,for example.

Next, a third electrode 143 is formed on third main surface 13. Thirdelectrode 143 functions as a drain electrode. Third electrode 143 iscomposed of an alloy containing nickel and silicon (for example, NiSi orthe like), for example.

Next, the dicing step (S24: FIG. 21) is performed. Silicon carbideepitaxial substrate 100 is divided into a plurality of semiconductorchips by being diced along dicing lines. Thus, silicon carbidesemiconductor device 300 is manufactured (see FIG. 24).

Although the method for manufacturing the silicon carbide semiconductordevice in accordance with the present disclosure has been describedabove by taking a MOSFET as an example, the manufacturing method inaccordance with the present disclosure is not limited thereto. Themanufacturing method in accordance with the present disclosure isapplicable to various silicon carbide semiconductor devices such as anIGBT (Insulated Gate Bipolar Transistor), an SBD (Schottky BarrierDiode), a thyristor, a GTO (Gate Turn Off thyristor), a PiN diode, andthe like, for example.

(Evaluation 1)

1-1. Fabrication of Samples

First, silicon carbide epitaxial substrates 100 in accordance withsamples 1 and 2 are prepared. Silicon carbide epitaxial substrate 100 inaccordance with sample 2 is manufactured using the manufacturing methodin accordance with the present embodiment. Specifically, silicon carbidelayer 20 is formed, with the number of rotations of the susceptor plate,the flow rate of silane, the flow rate of propane, and the C/Si ratiobeing changed from the time point (t8) to the time point (t9) as shownin FIG. 13. Specifically, from the time point (t83) to the time point(t84), the C/Si ratio changes from 0.9 (A1) to 1.0 to 1.1 (A2) (see FIG.13). In contrast, in the silicon carbide epitaxial substrate inaccordance with sample 1, silicon carbide layer 20 is formed, with thenumber of rotations of the susceptor plate, the flow rate of silane, theflow rate of propane, and the C/Si ratio being maintained to besubstantially constant from the time point (t8) to the time point (t9).Specifically, from the time point (t8) to the time point (t9), the C/Siratio is maintained at more than or equal to 1.5. It should be notedthat second main surfaces 12 of silicon carbide epitaxial substrates 100in accordance with samples 1 and 2 are planes having an off angle of 4°from the (0001) plane.

Next, the substrate processing step (S20: FIG. 21) described above isperformed on silicon carbide epitaxial substrates 100 in accordance withsamples 1 and 2. Thereby, 18 MOSFETs in the shape of chips aremanufactured from each sample.

1-2. Conditions for Experiment

Long term reliability of the silicon carbide semiconductor devices isevaluated by constant current TDDB (Time Dependent DielectricBreakdown). The environmental temperature is 25° C. The current densityis 20 mA/cm².

1-3. Result of Evaluation on Reliability of MOSFET

FIG. 25 is a Weibull plot showing constant current TDDB measurementresults. In FIG. 25, the axis of ordinates represents a cumulativefailure rate (F) plotted on Weibull probability paper, and the axis ofabscissas represents a charge-to-breakdown (Q_(BD)) [unit: C/cm²]. Thecharge-to-breakdown is a total amount of charge which has passed througha gate insulating film until a MOSFET breaks down. The larger thecharge-to-breakdown is, the higher the long term reliability is. In FIG.25, a plot group including square legends indicates the MOSFETsmanufactured from silicon carbide epitaxial substrate 100 in accordancewith sample 1. A plot group including rhombic legends indicates theMOSFETs manufactured from silicon carbide epitaxial substrate 100 inaccordance with sample 2.

As shown in FIG. 25, in the MOSFETs manufactured from silicon carbideepitaxial substrate 100 in accordance with sample 1, thecharge-to-breakdown at a cumulative failure rate (F) of about 63% (inother words, at a position of 0 on the axis of ordinates) is about 21C/cm². In contrast, in the MOSFETs manufactured from silicon carbideepitaxial substrate 100 in accordance with sample 2, thecharge-to-breakdown at a cumulative failure rate (F) of about 63% isabout 47 C/cm². The above result indicates that the long termreliability of the MOSFETs manufactured from silicon carbide epitaxialsubstrate 100 in accordance with sample 2 is higher than the long termreliability of the MOSFETs manufactured from silicon carbide epitaxialsubstrate 100 in accordance with sample 2.

(Evaluation 2)

2-1. Fabrication of Samples

First, silicon carbide epitaxial substrates 100 in accordance withsamples 3 to 6 are prepared. Silicon carbide epitaxial substrates 100 inaccordance with samples 3 and 4 are manufactured using the manufacturingmethod in accordance with the present embodiment. Specifically, siliconcarbide layer 20 is formed, with the number of rotations of thesusceptor plate, the flow rate of silane, the flow rate of propane, andthe C/Si ratio being changed from the time point (t8) to the time point(t9) as shown in FIG. 13. Samples 3 and 4 are manufactured under thesame conditions as those for sample 2. For silicon carbide epitaxialsubstrate 100 in accordance with sample 3, after silicon carbide layer20 is formed on silicon carbide single crystal substrate 10 by epitaxialgrowth, CMP (Chemical Mechanical Polishing) treatment is performed onsecond main surface 12 to planarize second main surface 12. For siliconcarbide epitaxial substrate 100 in accordance with sample 4, the CMPtreatment is not performed. In contrast, in the silicon carbideepitaxial substrate in accordance with sample 1, silicon carbide layer20 is formed, with the number of rotations of the susceptor plate, theflow rate of silane, the flow rate of propane, and the C/Si ratio beingmaintained to be substantially constant from the time point (t8) to thetime point (t9). For sample 5, from the time point (t8) to the timepoint (t9), the C/Si ratio is maintained at 1.3. For sample 6, from thetime point (t8) to the time point (t9), the C/Si ratio is maintained at1.9. It should be noted that second main surfaces 12 of silicon carbideepitaxial substrates 100 in accordance with samples 3 to 6 are planeshaving an off angle of 4° from the (0001) plane.

Next, the substrate processing step (S20: FIG. 21) described above isperformed on silicon carbide epitaxial substrates 100 in accordance withsamples 3 to 6. Thereby, 18 MOSFETs in the shape of chips aremanufactured from each sample.

2-2. Conditions for Experiment

Hazes in central regions 126 of second main surfaces 12 of siliconcarbide epitaxial substrates 100 in accordance with samples 3 to 6 aremeasured. The hazes are measured using SICA manufactured by LasertecCorporation, for example. The measuring method is as described above.The value of the haze of each sample is plotted on the axis of abscissasin FIG. 26.

Then, long term reliability of the silicon carbide semiconductor devicesis evaluated by constant current TDDB. The environmental temperature is25° C. The current density is 20 mA/cm². The cumulative failure rate (F)and the charge-to-breakdown (Q_(BD)) are plotted on a Weibull plot, asin FIG. 25. In the MOSFETs manufactured from silicon carbide epitaxialsubstrate 100 in accordance with each sample, the charge-to-breakdown ata cumulative failure rate (F) of about 63% (in other words, at aposition of 0 on the axis of ordinates) is determined, and thecharge-to-breakdown is plotted on the axis of ordinates in FIG. 26.

2-3. Result of Evaluation on Reliability of MOSFET

As can be seen in FIG. 26, the charge-to-breakdown becomes larger as thevalue of the haze becomes smaller. Even when the value of the hazebecomes more than 75 ppm, the charge-to-breakdown does not becomesmaller too much. In contrast, when the value of the haze becomes lessthan or equal to 75 ppm, the charge-to-breakdown sharply becomes larger.That is, it is conceivable that the reliability of an insulating film ofa MOSFET is significantly improved by setting the value of the haze toless than or equal to 75 ppm.

It should be understood that the embodiment disclosed herein isillustrative and non-restrictive in every respect. The scope of thepresent invention is defined by the scope of the claims, rather than theembodiment described above, and is intended to include any modificationswithin the scope and meaning equivalent to the scope of the claims.

REFERENCE SIGNS LIST

1, 2, 90: pit; 3: first straight line; 4: second straight line; 5: firstflat; 6: first plot group; 7: second plot group; 8: step-flow growthdirection (one direction); 10: silicon carbide single crystal substrate;11: first main surface; 12: second main surface; 13: third main surface;14: fourth main surface (surface); 20: silicon carbide layer; 23: firstlayer; 24: second layer; 25: threading dislocation; 26: bottom layerregion; 27: buffer layer; 28: drift layer; 29: surface layer region; 30:trapezoidal defect; 31: origin; 32: upper base portion; 33: protrudingportion; 34: base portion; 35: step bunching; 40: triangular defect; 50:bar-like pit; 51: first width; 52: second width; 60: circular pit; 70:triangular pit; 80: groove portion; 81: first groove portion; 82: secondgroove portion; 91: first arrow; 92: second arrow; 93: third arrow; 94:fourth arrow; 95: fifth arrow; 96: sixth arrow; 97: seventh arrow; 98:eighth arrow; 100: silicon carbide epitaxial substrate; 101: firstdirection; 102: second direction; 103: third direction; 104: fourthdirection; 111: maximum diameter; 121: central portion; 122, 126:central region; 123, 125: outer peripheral region; 124: outer edge; 127:intermediate region; 131: drift region; 132: body region; 133: sourceregion; 134: contact region; 136: oxide film; 137: interlayer insulatingfilm; 138: interconnection layer; 141: first electrode; 142: secondelectrode; 143: third electrode; 200: film forming device; 201: reactionchamber; 202: preheating structure; 204: quartz tube; 205: heatinsulator; 206: induction heating coil; 207: gas inlet; 208: gas outlet;210: susceptor plate; 211: first base member; 212: first coat portion;213: decomposition point; 220: heating element; 221: first region; 222:second region; 223: third region; 225: second base member; 226: secondcoat portion; 300: silicon carbide semiconductor device.

1. A silicon carbide epitaxial substrate, comprising: a silicon carbidesingle crystal substrate having a first main surface; and a siliconcarbide layer on the first main surface, the silicon carbide layerincluding a second main surface opposite to a surface thereof in contactwith the silicon carbide single crystal substrate, the second mainsurface having a maximum diameter of more than or equal to 100 mm, thesecond main surface including an outer peripheral region which is within3 mm from an outer edge of the second main surface, and a central regionsurrounded by the outer peripheral region, the central region having ahaze of less than or equal to 75 ppm.
 2. The silicon carbide epitaxialsubstrate according to claim 1, wherein the second main surface is a(0001) plane, or a plane inclined from the (0001) plane by less than orequal to 8°.
 3. The silicon carbide epitaxial substrate according toclaim 2, wherein in a direction parallel to the second main surface, aratio of a standard deviation of a carrier concentration to an averagevalue of the carrier concentration in the silicon carbide layer is lessthan or equal to 4%, and the average value is less than or equal to2×10¹⁶ cm⁻³.
 4. The silicon carbide epitaxial substrate according toclaim 2, wherein a groove portion is present in the second main surface,the groove portion extending in one direction along the second mainsurface, having a width in the one direction which is twice or more awidth thereof in a direction perpendicular to the one direction, andhaving a maximum depth from the second main surface of less than orequal to 10 nm.
 5. The silicon carbide epitaxial substrate according toclaim 4, wherein the groove portion includes a first groove portion, anda second groove portion connected to the first groove portion, the firstgroove portion is at one end portion of the groove portion in the onedirection, and the second groove portion extends from the first grooveportion along the one direction to reach the other end portion oppositeto the one end portion, and has a depth from the second main surfacewhich is smaller than a maximum depth of the first groove portion. 6.The silicon carbide epitaxial substrate according to claim 2, wherein apit originating from a threading screw dislocation is present in thesecond main surface, the pit has an area density of less than or equalto 1000 cm⁻², and within the pit, a maximum depth thereof from thesecond main surface is more than or equal to 8 nm.
 7. The siliconcarbide epitaxial substrate according to claim 6, wherein the pit has anarea density of less than or equal to 100 cm⁻².
 8. The silicon carbideepitaxial substrate according to claim 6, wherein the pit has an areadensity of less than or equal to 10 cm⁻².
 9. The silicon carbideepitaxial substrate according to claim 6, wherein the pit has an areadensity of less than or equal to 1 cm⁻².
 10. The silicon carbideepitaxial substrate according to claim 6, wherein, within the pit, amaximum depth thereof from the second main surface is more than or equalto 20 nm.
 11. The silicon carbide epitaxial substrate according to claim6, wherein the pit has a planar shape including a first width extendingin a first direction, and a second width extending in a second directionperpendicular to the first direction, and the first width is twice ormore the second width.
 12. The silicon carbide epitaxial substrateaccording to claim 1, wherein the second main surface is a (000-1)plane, or a plane inclined from the (000-1) plane by less than or equalto 8°.
 13. The silicon carbide epitaxial substrate according to claim12, wherein in a direction parallel to the second main surface, a ratioof a standard deviation of a carrier concentration to an average valueof the carrier concentration in the silicon carbide layer is less thanor equal to 5%, and the average value is less than or equal to 2×10¹⁶cm⁻³.
 14. The silicon carbide epitaxial substrate according to claim 13,wherein the ratio is less than or equal to 3%.
 15. The silicon carbideepitaxial substrate according to claim 13, wherein the ratio is lessthan or equal to 2%.
 16. The silicon carbide epitaxial substrateaccording to claim 13, wherein the ratio is less than or equal to 1%.17. The silicon carbide epitaxial substrate according to claim 12,wherein in the second main surface, trapezoidal defects, which aretrapezoidal depressions, have an area density of less than or equal to0.5 cm⁻², the trapezoidal defects each include an upper base portion anda lower base portion intersecting with a <11-20> direction when viewedin plan view, the upper base portion has a width of more than or equalto 0.1 μm and less than or equal to 100 μm, the lower base portion has awidth of more than or equal to 50 μm and less than or equal to 5000 μm,the upper base portion includes a protruding portion, and the lower baseportion includes a plurality of step bunchings.
 18. The silicon carbideepitaxial substrate according to claim 12, wherein, in the second mainsurface, triangular defects have an area density of less than or equalto 0.5 cm⁻².
 19. The silicon carbide epitaxial substrate according toclaim 12, wherein the silicon carbide epitaxial substrate has a bow ofless than or equal to 50 μm.
 20. The silicon carbide epitaxial substrateaccording to claim 1, wherein the maximum diameter is more than or equalto 150 mm.
 21. The silicon carbide epitaxial substrate according toclaim 1, wherein the silicon carbide layer has a thickness of more thanor equal to 5 μm.
 22. A method for manufacturing a silicon carbidesemiconductor device, comprising: preparing a silicon carbide epitaxialsubstrate according to claim 1; and processing the silicon carbideepitaxial substrate.